FIG. 10 is a schematic representation of prior art illumination system 700 for extreme ultra-violet (EUV). System 700 includes plasma source PS and mirror 702. Plasma from source PS generates EUV light L, for example plasma is generated by focusing a high power laser (not shown) onto a tin or xenon target. Mirror 702 is arranged to reflect and focus light L onto bundle 704 of light rays 706 focused on focus plane FP. That is, mirror 702 is arranged to reimage the plasma to plane FP. For example, system 700 can be part of a semi-conductor inspection system and optics 708 are located in FP and arranged to convert collected photons at plane FP into desired spatial and angular distribution at an illumination field (not shown).
Mirror 702 is configured for a specific size of the plasma and a specific angular distribution of the intensity of the plasma, which in turn results in a specifically sized space, for example in the D1 and D2 directions, for source PS, mirror 702, and plane FP. System 700 also includes non-tapered rectangular homogenizing tunnel 710 positioned at plane FP and used to homogenize EUV light from plane FP for use by optics 708. In general, it is desirable to maintain a uniform image size at plane FP, since changes in the image size result in undesired changes to the spatial and angular distribution at the illumination field.
Advances in semi-conductor inspection technology are resulting in changes to the size of the plasma or the angular distribution of the intensity of the plasma. In order to maintain the desired spatial and angular distribution at the illumination field when the plasma source is modified, the size of the image in plane FP must be preserved. To preserve the size of the image in plane FP, other portions of system 700 must be modified. In particular, the relative positions of source PS, mirror 702, and plane FP must be modified. In general, distance 712 must be expanded to accommodate the changes in position. However, expanding distance 712 results in increases complexity and cost, since space in semi-conductor inspection systems is tightly rationed. Thus, in the face of the changes to the plasma noted above, the previous optical and physical properties of system 700 cannot be simultaneously retained.